The present invention generally relates to an implantable cardiac stimulation device including both a ventricular defibrillator and a rate adaptable cardiac pacemaker. The present invention more particularly relates to such a device and method providing enhanced post-pacing sensing performance.
Combined implantable ventricular defibrillator and pacemaker stimulation devices are well known in the art. Such devices permit a heart to be paced for treating bradycardia, for example, while also detecting for ventricular fibrillation and ventricular tachycardia and applying defibrillating electrical energy, cardioversion shocks or antitachycardia pacing pulses to the heart when fibrillation or tachycardia is detected.
One problem that must be addressed in such devices is the need to provide relatively low threshold, i.e., high sensitivity, ventricular sensing for detecting fibrillation while pacing the heart. The sensing threshold must be low enough (sensitive enough) for detecting the low amplitude electrical activity of the heart during fibrillation while avoiding over-sensing which could result in a T wave being detected by the pacemaker and thus mistaken for an R wave. The foregoing is most notably a problem after a pacing stimulation pulse is applied to the heart by such devices.
In the prior art, post-pacing sensing has been performed by first establishing a ventricular refractory period (VREF) when the pacing stimulation pulse is applied and continuing the VREF for a pre-determined time through the evoked response. Following the VREF, the sensing threshold is set at an initial level, held at the initial level for a delay time, and then decreased thereafter from the initial threshold level to a minimum threshold level where it is held until the next paced or sensed event. The initial threshold, delay time and threshold decay rate are selected so that the threshold is above the amplitude of the T wave when the T wave occurs.
These post-pace sensing parameters can be varied to achieve the desired sensing threshold characteristics. For increased sensitivity to low level signals, as occur during fibrillation, it is desirable for the threshold to decrease to the minimum threshold as quickly as possible before the next pace pulse. However, to prevent over-sensing of larger T waves, particularly in patients with longer QT intervals, it is desirable for the sensing threshold to be higher or less sensitive. Therefore, the most optimal set of post-pace sensing parameters is the one which achieves the desired threshold level without over-sensing T waves. This problem is further complicated when rate adaptive pacing is implemented. Rate adaptive pacing is used with patients whose heart rates do not naturally increase in response to exercise (chronotropic incompetence). The rate adaptive pacer senses a physiologic parameter indicative of exercise and provides a corresponding increase in the pacing rate. However, this reduces the time between stimulation pulses and thus the time during which the sensing threshold can decrease to ensure the detection of low-level fibrillation signals. The time between pacing pulses is also shortened in a P-wave tracking mode for those patients whose hearts are not chronotropically incompetent. P-waves are sensed in the atria and the ventricle(s) is paced at the rate which tracks the P-waves and thus at a rate that may increase as a result of exercise or excitement. As used herein, the term xe2x80x9crate adaptivexe2x80x9d is intended to include pacing at a rate that varies in response to some change in physiological condition whether that be P-wave tracking, response to a sensor measuring exercise or otherwise. Further, since the QT interval generally shortens with faster pacing rates (and conversely lengthens with slower rates), a single post-pace sensing parameter set cannot yield the most optimal thresholding for all pacing rates. Another complicating factor is the variability of QT intervals and T wave amplitudes between patients and differing conditions. Prior art sensing systems have not addressed this problem of faster pacing rates in a rate adaptive pacer reducing the amount of time available for the threshold to decrease, compounding the problem of achieving the desired threshold (or sensitivity) by the next pace pulse.
The present invention addresses the problem of achieving the optimal thresholding during variable pacing rates. The present invention achieves the optimal thresholding without requiring complicated programming of the device by the patient""s physician.
The invention provides an implantable stimulation device including a ventricular defibrillator and a rate adaptive cardiac pacer which optimizes sensing performance following application of pacing pulses to a heart. The device includes a pulse generator that applies pacing stimulation pulses to a patient""s heart at a stimulation rate that is a function of physiologic demand. The device further includes a sensing circuit that senses ventricular activity of the heart for supporting pacing of the heart and fibrillation detection. The sensing circuit senses ventricular activity in accordance with a plurality of sensing parameters including post-stimulation sensing parameters. The device further includes a processor that adjusts the post-stimulation sensing parameters responsive to the stimulation rate.
The invention still further provides a method of applying stimulation pulses to a heart and sensing ventricular activity after applying a stimulation pulse to the heart. The method includes the steps of applying stimulation pulses to a heart at a stimulation rate that is a function of physiologic demand, sensing ventricular activity of the heart responsive to a plurality of sensing parameters including post-stimulation sensing parameters, and adjusting the post-stimulation sensing parameters responsive to the calculated stimulation rate.